1. Field of the Invention
The present invention relates to a scanning method for a disk player, and in particular to a scanning method whereby data recorded on a recording disk such as a video disk can be played back at high speed.
2. Description of Background Information
FIG. 1 is a general block diagram of an apparatus which has been proposed in the prior art for implementing scanning by a disk player, i.e. a high-speed playback operation function whereby so long as a predetermined command is being issued, high-speed playback is executed by successive repetitions of operations consisting of playing back a portion of the data recorded on a disk, jumping over one or more recording tracks of the disk, again performing data playback, and so on. In FIG. 1, reference numeral 1 denotes a photodetector which is contained within a pickup and utilized to detect tracking deviations of the pickup by the 3-beam method. Elements 1a and 1c of the photodetector 1 receive light which is reflected from a spot of light formed on the recording disk by an auxiliary light beam which is used for tracking error detection purposes. Output signals from the elements 1a and 1c are supplied to a subtractor circuit 2, whereby a tracking error signal b is produced. In addition, an element 1b of the photodetector 1 receives light which is reflected from a light beam spot (the data detection point) which is formed on the recording disk by a main beam and which is used for detection of data recorded on the disk. The resultant output signal from element lb is supplied to an FM demodulator 3 whereby a video signal e is reproduced.
The output signal from the subtractor circuit 2 is transferred through a loop switch 4 to an equalizer 5 which performs phase correction of the tracking error signal. The output signal a from the equalizer 5 is applied as a drive signal to a tracking actuator 6 to thereby complete a tracking servo loop. In addition, the output signal a from the equalizer 5 is supplied to a window comparator 7 and to a motor drive circuit 8. The window comparator 7 is configured such as to produce an output signal which is at a high level when the absolute value of the output signal level from the equalizer 5 is higher than a predetermined value. The output signal c from the window comparator 7 is supplied as a trigger signal to a monostable multivibrator 9. The Q output d from the monostable multivibrator 9 is used as a control signal which executes ON/OFF control of the loop switch 4. Specifically, when the Q output signal d is at the low level, the loop switch 4 is set in the OFF state (i.e. the open state).
The output signal from the motor drive circuit 8 is supplied to a slider motor 10, which drives a slider (not shown in the drawings) which carries the pickup and which is slidably movable along a radial direction of a recording disk. The motor drive circuit 8 detects an amount of displacement of the tracking actuator 6, based on the output signal from the equalizer 5, and drives the pickup radially across the disk by means of the slider motor 10, until the amount of positional displacement becomes zero, i.e. until the pickup becomes positioned at the center of a range of movement of the tracking actuator 6. In addition, the motor drive circuit 8 responds to a scan command by moving the slider radially across the disk towards either the outer or the inner periphery of the disk, at a fixed predetermined speed.
The operation of the various sections of this prior art example will be described referring to the waveform diagrams of FIGS. 2A to 2E. FIG. 2A shows the waveform of output signal a from the equalizer 5. FIG. 2B shows the waveform of the tracking error signal b, FIG. 2C shows the output signal c from the window comparator 7, FIG. 2D shows the Q output d of the monostable multivibrator 9, and FIG. 2E shows the waveform of the video signal e which is produced from the FM demodulator 3.
When a scan command is issued, the slider is moved for example towards the outer periphery of the disk by the motor drive circuit 8. When this occurs, the level of the output signal a from the equalizer 5, which is supplied as a drive signal to the tracking actuator 6 increases at a rate whose slope is in accordance with the velocity of displacement of the slider. As a result, the amount of displacement of the tracking actuator 6 becomes large. However when the absolute value of the output a from the equalizer 5 reaches a predetermined value, the output signal c from the window comparator 7 goes to the high level. When this occurs, the monostable multivibrator 9 is triggered, whereby the Q output d thereof goes to the low level during a time interval T.sub.1 which is determined by the time constant of monostable multivibrator 9. As a result, the loop switch 4 is held in the OFF state during the time interval T.sub.1, and hence the tracking servo loop is held in the open state. As a result, the position of the tracking actuator 6 is restored to the center of the range of movement thereof. In this way, the data sensing light spot of the pickup is displaced by jumping over a number of recording tracks.
When the monostable multivibrator 9 switches back to the stable state thereof, so that the loop switch 4 is again closed, the tracking servo loop is thereby closed and hence the data sensing light spot of the pickup again becomes positioned above a recording track so that read-out of data from the track can begin.
High-speed data playback can be performed by the operation described above. However, while the loop switch 4 is in the open state, the video signal e produced as output from the FM demodulator 3 does not provide a normal display of data, and noise is produced within that signal. In order to reduce the effects of such noise, the period of repetition of the time intervals (T.sub.2) during which the tracking servo loop is held in the open state can be made of sufficient
duration by suitably setting the value of the reference level of the window comparator 7. However if this is done, then it is necessary to extend the range of movement of the actuator, and the field of view of the pickup. In addition, when the tracking servo loop is in the open state for too long duration, a large number of tracks are jumped over by the data sensing light spot of the pickup, so that continuity of the playback image is not maintained, and it is difficult for the user to search for a desired display image.
Furthermore, it is difficult to adjust the speed of such high-speed playback, due to manufacturing deviations in the characterisitics of the slider motor.
Moreover in the case of a video signal which is read out from a memory, the frequency range of the playback signal is more limited than that of the original signal which is written into the memory, with this range of frequencies which can be played back being dependent upon the sampling period. For this reason, it is undesirable to utilize storage in a memory, since this is disadvantageous with regard to attaining high resolution of a display image.